http://leader.pubs.asha.org/article.aspx?articleid=2278314Advances in Audiology Research: An OverviewAt a time when we are halfway through the first decade of the 21st century, our knowledge of hearing and hearing loss still is making important advances. This progress extends across many areas of clinical audiology. The area of implanted auditory prostheses including cochlear and brainstem implants and bone-anchored hearing ...2006-03-01T00:00:00FeaturesBrenda Lonsbury-Martin

At a time when we are halfway through the first decade of the 21st century, our knowledge of hearing and hearing loss still is making important advances. This progress extends across many areas of clinical audiology.

The area of implanted auditory prostheses including cochlear and brainstem implants and bone-anchored hearing aids is making significant progress. After many years of experience, technical advances in design, improvement in stimulation strategies, and progressive changes in criteria for implantation, electrical stimulation of the auditory pathway in patients with significant sensorineural hearing loss of cochlear and retrocochlear origin remains an effective and safe treatment.

Evoked Response Findings

Some important new knowledge from comparing electrically evoked response findings between patients with different etiologies has important implications for developing optimal implantable devices. For example, a comparison of the pathologies involved in both connexin 26- (CX26) related deafness, which involves dysfunction of specialized intercellular gap-junction channels that transport potassium within the inner ear, and non-CX26 deafness suggests more consistent spiral ganglion cell survival along the length of the cochlea in the CX26 disorder. In contrast, non-CX26-related deafness appears to involve a decreasing gradient of spiral ganglion cell survival from the apex to the base of the cochlea. Such detailed knowledge about underlying anatomical damage associated with a particular hearing loss implies the possibility of customizing implantable devices for patients.

Further research on various types of bone-anchored hearing aids (BAHAs) confirms that they are a valuable alternative to conventional air and bone-conduction hearing aids, particularly for patients with conditions such as chronic otitis media or bilateral aural atresia. Not only does the BAHA have great cosmetic acceptability and comfort, but it has demonstrated better sound quality that can lead to improved speech comprehension.

Age-Related Deficits

The number of cases of presbycusis is rising as the median age of the population advances. Questions remain about whether the age-related deficits in hearing are primarily a result of reduced peripheral auditory sensitivity. Evidence is increasing that psychoacoustic abilities, such as temporal processing of brief acoustic sounds, slow significantly with advancing age. Such data support the notion that a major contributor to poor speech understanding by older individuals may involve defective processing at the central auditory nervous system. Thus, simple amplification strategies that compensate for loss of sensitivity by the peripheral auditory system may not easily combat communication problems commonly exhibited by the elderly. An emerging concept about the aging ear indicates a link between an individual’s vulnerability to noise damage and genetically determined susceptibility. The amount of sub-lethal noise damage to a given ear determines the degree and rate of development of presbycusis for that ear or, at the very least, exacerbates it and increases the ear’s vulnerability to aging.

Newborn Hearing

Important information in the newborn hearing screening area emerged recently from an examination of the efficacy of a two-stage protocol incorporating evoked otoacoustic emissions (OAEs) as the initial screen with follow-up of failures using automated auditory brainstem responses (A-ABRs). Many of these infants, who failed OAEs but passed the A-ABR at the newborn-screening stage, exhibited a permanent hearing loss by age 1. These results suggest that monitoring hearing beyond birth is an important component of the early detection of hearing loss. Such significant findings argue for neonatal screening for connexin-based gene mutations to aid early intervention in cases that will eventually exhibit late-onset deafness. The findings have implications for public-health principles of screening programs, and call for research to understand the basis of such late-onset hearing impairments.

Irreplaceable Hair Cells

Contemporary research efforts involving molecular biology have provided new insights into the fundamental processes underlying common hearing problems involving the death of irreplaceable hair cells, including noise-induced hearing loss (NIHL) and ototoxicity. An understanding of the pivotal role of the stress-induced generation of reactive oxygen species and how such free radicals can damage cochlear tissues and lead to cell death has spawned an industry of pharmaceutical interventions based on naturally occurring and synthetic antioxidants. In addition to antioxidants, other promising new otoprotectants are being fashioned to interrupt the biochemical pathway that leads to stress-induced cell death. Research shows that susceptibility to NIHL or ototoxicity varies across different individuals because of the idiosyncratic interactions of intrinsic genetic and extrinsic environmental factors. Interventions that target the prevention of stress-induced cell death promise to reduce the adverse effects of excessive sounds and ototoxic compounds on hearing.

Brain imaging techniques such as magnetoencephalography and functional magnetic resonance imaging are advancing in methodological complexity. For example, specialized pre- and post-processing scanning techniques have been developed that can be adapted to 3 Tesla imaging systems that are common to clinical setups. This approach permits the activation of subcortical auditory structures by controlling cardiac-related pulsatile movements of both the brainstem and the cerebral spinal fluid. Functional brain mapping has led to insights about the connectivity of structure and function within the auditory system as well as to the localization of cognitive processes. Functional neuroimaging also suggests the possibility of more effective treatments for such puzzling otological symptoms as hyperacusis and certain types of tinnitus. Other potential breakthroughs related to this powerful tool most likely will involve the acquisition of knowledge about the plasticity of the central auditory system’s compensatory reaction to such dysfunctions.

The discovery more than two decades ago that hair cells can regenerate in birds and other non-mammalian vertebrates produced a wide range of studies designed to uncover ways of restoring hearing and balance after damage from aging, genetic defects, and environmental stresses such as excessive noises or chemotherapeutic drugs. One of the most exciting areas involving hair-cell regeneration concerns regeneration of hair cells with the use of stem cells. The discovery of both adult and embryonic stem cells in the cochlea and evidence that they can be converted into hair cells raise hope for the development of stem-cell based treatment regimes to regenerate damaged cochlear components.

Other Research Areas

Other study areas in today’s audiology research arena concern the use of auditory steady-state responses as predictors of behavioral hearing thresholds, particularly in difficult-to-test infants and young children; the development of high-frequency OAE tests; the application of vestibular evoked myogenic potentials toward a better understanding of balance problems; and power reflectance measures of sound transmission through the middle ear.

Such investigations represent only a few topics under active study. Most certainly, evidence-based practice approaches based on, for example, randomized controlled trials such as that used recently to demonstrate the efficacy of tinnitus treatment using habituation and tinnitus retraining therapy are moving research findings quickly from the laboratory bench to the clinic. Together, basic, translational, and experimental clinical research are benefiting the screening, diagnosis, and habilitation of hearing problems by hearing healthcare practitioners.